1. Field of the Invention
The present invention relates to a solid state radiation sensor, suitable for use with an imaging apparatus using radiation, such as X-rays or the like, and a manufacturing method of the sensor.
2. Description of the Related Art
In the field of medical X-ray imaging, X-ray imaging panels (solid state radiation sensors) are known. The X-ray imaging panel uses an X-ray sensitive photoconductive layer as a photoconductor, in order to improve diagnostic capabilities with a reduced amount of radiation received by a subject, and an electrostatic latent image formed by the X-rays on the photoconductive layer is read out by light or multitudes of electrodes and recorded. The method described above is superior to the known image projection method called indirect imaging using TV camera tube, in that it may obtain higher resolution.
The X-ray imaging panel described above includes therein a charge generation layer that receives X-rays and generates charges corresponding to the received X-ray energies, and the generated charges are read out as electrical signals. The photoconductive layer described above functions as the charge generation layer. Generally, materials such as amorphous selenium (a-Se), PbO, PbI2, HgI2, BiI3, Cd(Zn)Te, and the like are used for the photoconductive layer.
The use of amorphous selenium may readily provide large surface area through a thin film forming technique such as vacuum deposition method and the like, but the resultant photoconductive layer have many structural defects because of its amorphous nature, so that the sensitivity of the photoconductive layer is likely to be degraded. Consequently, it is customary that an appropriate amount of impurity is doped in the amorphous selenium in order to improve the performance. For example, U.S. Patent Application Publication No. 20030223534 describes a recording photoconductive layer formed of amorphous selenium doped with an alkali metal of 0.01 to 10 ppm. Japanese Unexamined Patent Publication No. 2001-244492 describes a recording photoconductive layer formed of amorphous selenium doped with Na of 70 ppm as the alkali metal. Further, U.S. Pat. No. 3,685,989 describes a recording photoconductive layer formed of amorphous selenium doped with Na of 100 ppm.
Further, U.S. Pat. No. 5,396,072 describes an X-ray image detector that includes a semiconductor layer with a thickness in the range from 0.5 to 2 μm formed of selenium doped with an alkali metal of 20 to 200 ppm that conducts electrons without conducting holes. Still further, the Canadian Patent No. 2184667 describes an X-ray image multilayer plate that includes a semiconductor layer having unipolar buffer layers, each with a thickness of 0.5 and 10 μm formed of selenium doped with an alkali metal.
As a result of committed study, the inventors of the present invention have found that the electron mobility is improved if a predetermined region between electrodes of the solid state radiation sensor has a higher alkali metal density than the other region. The amorphous selenium doped with an alkali metal like that described in U.S. Patent Application Publication No. 20030223534 may cause local crystallization, and the use of such amorphous selenium for the photoconductive layer may cause an image defect with time.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a solid state radiation sensor that includes a recording photoconductive layer having a predetermined density distribution of a doped alkali metal, less structural defects, and high electron mobility, and a manufacturing method of the solid state radiation sensor.